MULTIPLE A-TYPE CYCLINS EXPRESS SEQUENTIALLY DURING THE CELL-CYCLE INNICOTIANA-TABACUM BY2 CELLS

Four full-length and one partial cDNA clones encoding four different A -type cyclins were isolated from a tobacco S-phase-specific library. T he corresponding mRNAs displayed sequential appearance and disappearan ce during the cell cycle of highly synchronized suspension-cultured to bacco cells. Sequence analysis showed that the plant A-type cyclins ca n be subdivided into three distinct structural groups that are likely to be represented in every plant species. Two of the isolated tobacco cyclins belonging to the same group were highly expressed throughout S and G(2) phases but showed different kinetics of induction at the G(1 )/S transition. Another one belonging to a second group was induced at mid-S phase and expressed until mid-M phase. A similar expression pat tern was previously reported for a tobacco cyclin belonging to the thi rd group. This sequential expression of multiple A-type cyclins in one type of plant cells makes a clear distinction from the situation in a nimal cells in which only one A-type cyclin exists in a given species. Furthermore, the expression of the different A-type cyclin genes resp onded differently upon a block at mid-S phase by DNA synthesis inhibit ors. These results suggest that the multiple A-type cyclins act at dif ferent steps of the plant cell cycle and, therefore, exert distinct fu nctions. In contrast, the expression of B-type cyclins,vas restricted to a narrow window corresponding to the M phase.